The use of electrooptic modulators for high dynamic range fiber optic link applications in the communications field and in other applications has been hindered by the nonlinearity of these devices. The Mach-Zehnder modulator which is the work horse of the analog links, possesses an intrinsic nolinearity due to the interferemedic nature of its operation. Linearization of these devices has become necessary in the most demanding applications. The linearization of electrooptic modulators for high dynamic range applications has taken many forms.
Basically, there are two classes of linearization that have been explored. The first class contains those devices having electronic predistortion which electronically produces the correction for third order distortion prior to feeding the signal into a modulator. This approach is limited in its capacity to maintain the proper amount of correction over very large signal applications where higher order nonlinearities occur. In addition, the stability required in most applications is lacking and devices incorporating the electronic predistortion are difficult to produce.
The other approach which has been explored on a limited basis is founded on classic feed-forward techniques used in the design of high performance, high frequency RF amplifiers for many years. In this approach, the nonlinear element is the modulator and is driven directly by the supplied RF signal. The output of this nonlinear element is then electronically compared to the input, and an error signal is produced. The error signal is simply the difference between the input to and output of the nonlinear element. This error signal is then amplified, fed forward and combined with the output of the first nonlinear element. Care has to be taken to match the amplitude and phase of the error signal and the original output of the element in order to insure adequate cancellation of the error signal.
It would be desirable to have a system for correcting harmonic distortion in fiber optic networks which is wavelength independent and which allows the use of a single light source. The present system is drawn towards such an invention.